This invention relates to a process for preparing ethane derivatives containing the trifluoromethyl group corresponding to the formula CF.sub.3 CHYZ, in which Y and Z may be the same or different and are hydrogen, chlorine or bromine or fluorine.
Perhalogenated organic compounds which are used as refrigerants, degreasing agents, lubricants and release agents or in the production of plastic foams, are increasingly being replaced by hydrogen-containing halogenated organic compounds.
One class of compounds highly suitable as substitutes for perhalogenated organic compounds is that of trifluoromethyl-containing ethane derivatives corresponding to the formula CF.sub.3 CHYZ in which Y and Z may be the same or different and are hydrogen, fluorine, chlorine or bromine.
Compounds of this type can be prepared by halogen/fluorine exchange which is usually a chlorine/fluorine exchange. One disadvantage of known preparation processes is the poor yield. Furthermore, undesirable perhalogenated by-products are often also formed.
However, the industrial preparation of these compounds is not without problems. The reason for this is that the more highly halogenated compounds are relatively inert towards chemical reactions.
Processes for preparing CF.sub.3 CHCl.sub.2 are known. One preparation of CF.sub.3 CHCl.sub.2 in the liquid phase is described in A. E. Feiring, J. Fluor. Chem., Vol. 13, pp. 7-18 (1979). There, fluorotetrachloroethane is reacted with hydrogen fluoride in the liquid phase in the presence of tantalum pentafluoride catalyst. The yield of CF.sub.3 CHCl.sub.2 is about 10%. Starting from tetrachloroethylene, the preparation is only possible with one single catalyst, namely again with tantalum pentafluoride, but in a yield of only 2%.
A gas phase process considered the only industrially useful process for preparing CF.sub.3 CHCl.sub.2 is described European Patent Application No. EP 282,005. The preparation of CF.sub.3 CHCl.sub.2 is carried out at temperatures of 320.degree. to 360.degree. C. by reacting tetrachloroethylene with hydrogen fluoride on aluminum fluoride/chromium oxide catalysts. The degree of conversion and yield in this process are unsatisfactory, and the energy consumption naturally very high.